Conventionally, an electrolysis vessel for production of oxidized water for sterilization use is a two-chamber-type electrolysis vessel shown in FIG. 1 or a three-chamber-type electrolysis vessel shown in FIG. 2. In the two-chamber-type electrolysis vessel, a chlorine ion-dissolved water is supplied to an anode chamber by adding halogen salts such as salt to raw water. For example, when a salt solution is supplied, chlorine ions are oxidized to produce hypochlorite ions.
Specifically, in an oxidization reaction, chlorine ions are electrolytically oxidized to produce chlorine molecules. The chlorine molecules change in water into hypochlorite ions and hydrogen ions. Therefore, strongly acidic oxidized water is formed.[Chemical Formula 1]2Cl−-2e2Cl2  (1)2HClO+2H+Cl2+2H2O  (2)
When the two-chamber-type electrolysis vessel is used, the ion concentration is insufficient. In order to promote the electrolytic oxidation reaction, it is necessary that the electrolysis voltage be enhanced or halogen salts such as salt be added to raw water. When halogen salts are added, highly acidic oxidized water is produced. Therefore, there is a problem of strong acidity in terms of maintenance and control of a device.
FIG. 2 shows the three-chamber-type electrolysis vessel in which an intermediate chamber is provided between an anode chamber and a cathode chamber. In a partitioning membrane, an ion exchange membrane is provided, and a porous electrode shown in FIG. 3 is used as an electrode. A salt solution or the like is supplied to the intermediate chamber. Chlorine ions in the intermediate chamber are transferred to the anode chamber. Part of the chlorine ions forms chlorine molecules in accordance with a chemical formula (1) to finally form hypochlorous acid that is strongly acidic. When an aqueous solution such as a salt solution is added to the intermediate chamber of the three-chamber-type electrolysis vessel and sterilization water containing residual chlorine is produced, there have been the following problems.
(i) The solution with which the intermediate chamber is filled is strongly acidic, i.e., as high as a pH of 1 or less, and the maintenance of the device is difficult.
(ii) Oxidized water that is strongly acidic with a pH of 2 to 3, and produces conventional oxidized water for sterilization use is produced.
(iii) When a general three-chamber-type electrolysis vessel is used, a main oxidizable substance is the hypochlorite ion. There have arisen problems in which the hypochlorite ion has defects in which the sterilization effect is lost by pH of alkalinity and the sterilization effect is decreased in the coexistence of organic substances.
The three-chamber-type electrolysis vessel provided with the intermediate chamber has had the problems described above. For example, when a salt solution is added to the intermediate chamber, chlorine ions are transferred to the anode chamber, and sodium ions are transferred to the cathode chamber. It is known that the pH in the intermediate chamber becomes acidic due to a difference in ionic permeability between partitioning membranes. By long-time electrolysis, the pH of liquid in the intermediate chamber may become strongly acidic to decrease the pH to 1 or less.
Such a strongly acidic liquid has corrosiveness, and adversely affects equipment.
As described above, oxidized water containing residual chlorine has conventionally been produced by electrolytic oxidation using a liquid of halogen salts such as a salt solution. In terms of maintenance and control of the device, there have arisen problems such as corrosion due to hydrochloric acid. In order to solve the problems such as corrosion, a first object was to produce sterilization water by electrolytically oxidizing natural water such as typical tap water without adding halogen acid.
It has been reported that hypochlorite ions exhibit sterilizing power and contribute to the residual chlorine concentration. The hypochlorite ions have the following disadvantages.
(1) The sterilizing power in a neutral pH range of 4 to 6 is maximum, the sterilizing power in an alkaline pH range of 8 or more is largely decreased, and the sterilizing power at a pH of 10 or more is substantially eliminated.
(2) When the hypochlorite ions coexist with bacteria and organic substances, the sterilizing power is known to be decreased.
A second object was to enhance the sterilizing power and maintain the sterilizing power in an alkaline range, and further to produce an oxidizable substance, which does not largely decrease the sterilizing power even in the coexistence of organic substances, by electrolysis.
An oxidizable chlorine compound that can be produced by electrolysis is chlorine dioxide in addition to hypochlorite ions. The following document has reported that the sterilizing power of chlorine dioxide is constant in a pH range of 6.0 to 10.0, and is stronger than that of hypochlorous acid. (Masahiko Takayama, et al., J. Antibact, Antifung, Agent VOL. 23, No. 7, pp. 401)